Hello all,
Was sitting here thinking about a problem with HB amp I built about 4 years
ago, which still perplexes me. Put is aside at the time, figuring I would
take a fresh look at it when I got the time. Well, never found the time....
This is a pair of old, well worn 4-400's in a grounded grid configuration,
mono band for 10 meters. Plate voltage is about 4400 no load, with about
4000 or so loaded, as I recall.
Plate circuit is very compact with few, if any long leads. Most things are
directly connected to each other. Used #2 solid copper for the L coil and
vacuum and air caps for C1, C2.
Tuned input using 2, itty-bitty, end mounted, nickel plated, padder type
air variables and a toroid core. Forget the core #, but it was 'black' and
recommended to me by, Frank, W3LPL They are located right at the tube
sockets, so no long leads
Everything looks as I would expect it with a grid dipper , or my Autek box.
Amp is stable, and makes power.
Here is the 'hitch in the get-a-long':
(With plate current dipped) As I increase drive, the grid current goes
up. I tweak the C2 cap, grid current goes down; RF output goes up. Maybe
touch up the 'tune' cap a bit. All as expected.
At some point, when drive is increased and grid current goes up, no
adjusting of the C2 cap will cause this to be transferred and seen in the
RF output.
In the past, as probably most of have seen, this condition happens when you
run out of C2, at which point you would parallel a bit of extra fixed
capacitance, but I have plenty of C2
I can not make the extra RF on the input, be amplified and appear on the
output. Drive goes up; grid current goes up; tubes go RED; output stays
the same.
Driving it with about 100 w.
These same two tubes put out 1200 watts on 10 meters for years in another,
band switched, amp at 55-60% efficiency. All I can get in this setup is
about 850-900 watts out. Same power supply also (same voltages.)
Being the tubes are old (paid $20 for the pair in 1981 as 'pulls' from
commercial station in area) I've always needed to drive the grid harder,
running 250-300 mils, but current can be driven to 400 ma when the above
happens. In any case, I've gotten my money's worth from these two tubes.
Everything else seems to work so well, could I be doing something like
saturating the core on my input? It is wound with something like #16, or
maybe #18.
(Disclaimer: Understand, I am not an 'electronics professional', so don't
bust my chops if I have taken liberties with technical terms and concepts.)
Any help, or comments, from the 'gifted gray matter' out there would
appreciated.
73 de Steve, NR4M
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